1. Field of the Invention
The present invention relates generally to dimmable light emitting diode (LED) lamps, and in particular to an LED lamp power source that controls dimming of the LEDs from the secondary side of a transformer.
2. Background of the Invention
Lighting control and power supply integrated circuits (ICs) are in common use in both electronic systems and in replaceable consumer lighting devices, e.g., light-emitting-diode (LED) and compact fluorescent lamp (CFL) replacements for traditional incandescent light bulbs.
The power conversion required for operating LED lamps efficiently is typically from a relatively high-voltage rectified AC line power supply (e.g., 120 VAC or 240 VAC rectified to 180 VDC or 360 VDC), to the forward voltage drop of one or more LEDs arranged in a series-connected “string”, which is on the order of 5V-15V for the typical incandescent bulb replacement device. Since, without filtering, this rectified line voltage will vary at a slow rate (e.g., 120 Hz), energy must be stored to avoid varying the current supplied to the LEDs. Therefore, capacitors of sufficient storage must be provided to filter the rectified line voltage, and/or the rectified line voltage must be converted to a lower DC voltage to provide the proper operating voltage for the LEDs. Thus it is desirable to convert the rectified line voltage to a lower DC voltage using a transformer-coupled topology, such as a flyback converter.
However, by using a transformer-coupled topology, controlling the dimming level in dimmable LED lighting devices typically requires another isolated signal path, such as an optical isolator or signal transformer to permit communication of the dimming information (i.e., the shape of the AC line waveform that is provided from a thyristor-based dimmer) to the secondary side of the transformer, which raises the cost of the replacement lighting device, as well as complexity of the circuit.
Therefore, it would be desirable to provide a lower-cost transformer-isolated power source circuit that can supply LEDs without requiring a separate isolated signal path for controlling the brightness of one or more LED strings in conformity with a dimming level determined from the shape of the input AC line voltage.